The pigs need time to adjust. They're very cautious about what they eat. It can just take a lot of time for them to learn that they aren't going to die from eating these very large, strange things.
Perhaps they're just very, very picky pigs and they don't like what you're giving them. I'm sure you've already perused this, but Ly's thread on what to feed guinea pigs is great:
http://www.guineapigcages.com/forum/...arts-info.html Try your pigs out with different types of veg and see if they like those any better.
Otherwise I think you just need to be patient, offer a varied diet, and hope for the best. If you're very concerned about the vitamin issue, you could feed them a Vitamin C tablet. In the thread above Ly recommends 25-50mg daily. I've found the easiest way to do this is to grind the correct amount of an easily-available vegan (IE: no gelatine etc) Vitamin C tablet with the tiniest bit of water to make a paste, and let them lick it off my (very clean) fingers. Goes down a treat with my boys.
Hope that helped a little.